The present invention relates to a degassing centrifugal apparatus such as a pump and to a process for centrifugal degassing a liquid, especially backwater in the production of paper or board in a paper machine.
Backwater drained through a forming fabric in a papermaking process, normally, contains a large amount of enclosed air. Since the short circulation of a paper machine requires a particularly constant flow, disturbing air is normally removed by conducting the drained backwater, by means of special pipe or channel systems, from the dewatering box or boxes to an open backwater tank. The deaerated water is then pumped back to the fiber process of the short circulation preceding sheet forming. Other liquids which require air free pumping are, among others, black liquor, deinking sludge and stock, coating colour, etc.
Pumps, which are able to separate gas from a fluid to be pumped are well known as such, but the objective of such pumps is normally just to remove a sufficient portion of the gas to enable regular pumping. The known pumps are normally not capable of removing enough gas for achieving the degree of freedom of airs which is required for using the fluid directly for example in a papermaking process without further deaeration.
Examples of prior art pumps capable of uniformly pumping fluids which contain gas are disclosed in Patents such as U.S. Pat. No. 4,410,337 and U.S. Pat. No. 5,039,320. Said pumps are so called MC-pumps developed for pumping high consistency (about 5% to 20%) pulp which must be fluidized in the suction channel of the pump, whereby air is separated through shear and centrifugal forces. The separated air concentrates in the center of the pump and is discharged by various means. Due to the small separation volume and high viscosity of the fluids to be pumped the separation of liquid and gas is not complete in the prior art pumps mentioned above. Consequently, separation of solid material and liquid from the discharged air is further required as taught for instance in Patent Applications EP 337394 and EP 298442.
International Patent Application published as WO 92/03613 discloses a stock feeding arrangement and process wherein a fiber suspension is pumped by means of xe2x80x9cmodifiedxe2x80x9d versions of the MC-pumps mentioned above. However, the specification does not explain how these pumps are to be modified.
Other means for separating gas from fluids, or for pumping fluids containing or developing vapor are disclosed in Patents such as U.S. Pat. No. 3,203,354, U.S. Pat. No. 3,323,465, U.S. Pat. No. 3,856,483, U.S. Pat. No. 4,201,555, U.S. Pat. No. 3,973,930, U.S. Pat. No. 4,516,987, U.S. Pat. No. 4,600,413, U.S. Pat. No. 4,675,033, U.S. Pat. No. 4,908,048 and WO 93/23135.
The same applicant""s U.S. Pat. No. 5,861,052, the disclosure of which is included herein by reference, discloses a gas separating pump capable of separating air and water from a mixture thereof. The pump has a fluid inlet at one end and a pumping liquid outlet at the opposite end. Between inlet and outlet there is a hollow elongated gas separating rotor and a generally central outlet for separated gas. At the outlet end of the pump the diameter of the rotor increases smoothly to form a larger diameter pumping zone. The inlet end of the pump is provided with a set of blades for distributing the incoming fluid to the rotor walls and the outlet of the pump is provided with a blade wheel for pumping the degassed liquid. The apparatus is especially well suited for the gas-free re-cycling of backwater drained through a forming fabric in a papermaking process.
A variant of the above mentioned degassing pump is described in the same applicant""s patent application WO 96/19276 wherein a threshold means is provided for ascertaining that the liquid flow at the outlet end is in a tranquil flow mode. The threshold means defines the position at which the transition from rapid to tranquil flow will take place via a hydraulic jump. The hydraulic jump is described with reference to the Froude number (Fr), for instance by B. S. Massey; Mechanics of Fluids, 2nd Edition, Van Norstrand Reinhold Co. Ltd. London 1971, pp 334 to 359.
If the liquid flow in the gas separation drum of a centrifugal gas separation device is rapid at any stage, it should preferably be transformed into a tranquil state before being removed from the device in order to avoid excessive turbulence and re-entraining of gas into the liquid at the outlet end. This is particularly important in cases, where the objective is to produce a liquid essentially free of gas, as is the case for instance in the air-free pumping of back-water of a paper machine.
In many prior art pumps the axial flow is either so slow, i.e. the capacity of the pump so low, that tranquil flow prevails, or the need for a gas-free state is so secondary, that a rapid flow at the outlet can be accepted.
In a centrifugal apparatus like the one of the present invention, the rotational energy contained in the rotating liquid is used to separate the air contained in a fluid so as to provide a gas-free liquid rotating on the wall of the apparatus. The rotational energy is retained in the liquid and is used for pumping the liquid further from the apparatus. However, in some cases the rotational speed is very high and the pumping pressure may be larger than desired at the outlet end. Moreover, a high rotational velocity may cause increased energy losses in the apparatus.
An object of the present invention is to improve the function of known processes and apparatuses and to provide a means for adjusting the pressure of a liquid discharged from a centrifugal degassing apparatus at a high flow rate. The object of the invention is also to reduce the energy losses in the process. Another object of the invention is to design a rotatable degassing apparatus wherein a major portion of the energy is recovered as useful energy. It is a special object of the invention to control the hydraulic conditions in a degassing apparatus having a rapid flow of fluid.
The present invention is based on the realization that the energy losses in a degassing centrifugal apparatus may be reduced and that the rotational energy can be recovered by providing a turbine at the outlet. The unique features of the present invention are defined in the appended claims.
Thus, the present invention relates to a degassing centrifugal apparatus comprising a rotatable hollow rotor connected to a fluid inlet at one end and a liquid outlet at the opposite end, and having a gas exhaust connected to the center thereof, said apparatus having at its inlet end means for bringing a fluid to rotate on the inner wall of said rotor. According to the invention the apparatus comprises a turbine at its outlet end.
The turbine is configured to recover at least a portion of the dynamic energy contained in a fluid flowing in said rotor. The turbine preferably forms part of the rotor structure and is rotatable with the rotor.
The apparatus of the present invention is preferably used as a pump and the outlet end of the apparatus therefor preferably comprises a stationary spiralled peripheral outlet for pumping the degassed liquid. When a pumping action is desired in addition to the degassing, the turbine is preferably located in a pumping zone formed in the rotor at its outlet end. The pumping zone preferably contains a turbine wheel extending into a stationary liquid outlet.
The turbine may have more or less distinct turbine shovels for recovering dynamic energy from the rotating liquid. In a preferred embodiment of the apparatus according to the invention the turbine wheel comprises shovels forming between themselves arcuate channels which are narrower at the outer periphery than at the inner periphery hereof.
The inlet end and the rotor body of the degassing apparatus of the present invention may be designed in various ways. It is important, however, that the fluid which is to be degassed is brought to rotate in the rotating rotor and is brought to flow axially along the rotor wall towards the outlet end and into the turbine of the present invention. The distribution of the fluid towards the rotor wall may be provided, for instance, by throttles, blades, tangential introduction of fluid, etc.
The main body of the degassing apparatus may be configured in accordance with the degassing pump described in the above mentioned U.S. Pat. No. 5,861,052 with its enlarged rotor outlet However, other designs of the apparatus are also acceptable. The degassing portion of the rotor body may be tubular or conically widening towards the outlet. The rotor body may be provided with blades or ridges along its length, although a smooth rotor wall is preferred. The outlet end of the rotor should-preferably widen in a smooth and even way towards a peripheral outlet. An abrupt transformation from degassing zone to pumping/turbine zone may, however, also be accepted provided that the degassing capacity required for any given purpose is not impaired. A final degassing action may, for instance, take place in a rotating liquid ring at the outlet.
The rotor body is preferably elongated, i.e. its length preferably significantly exceeds its width. However, a satisfactory degassing effect for many purposes may be obtained with short and wide pumps since a large diameter increases the centrifugal forces and provides a good degassing effect and the excess dynamic energy of the liquid can be recovered with the turbine according to the present invention.
In a special embodiment of the present invention the inlet means are configured as blades and/or shovels which deflect and accelerate the fluid velocity to provide a higher rotational velocity and a lower axial velocity. The turbine of the present invention is especially advantageous for the recovery of the rotational energy in this kind of embodiment.
A high peripherical velocity of the rotating fluid will give rise to a high pressure at the outlet end. The pressure may be too high for pumping and in any case in excess of what is required. The present invention aims at recovering a part at least of the excess energy and enabling its use for other purposes such as for driving the rotor.
Thus, the present invention relates to a process for degassing a fluid by centrifuging, comprising feeding a fluid containing a mixture of liquid and gas into an inlet of a rotating hollow rotor; bringing said fluid to rotate on an inner wall of said rotor; bringing said fluid to flow axially towards a liquid outlet at the opposite end of said rotor while causing said gas to separate from said liquid; directing the resulting degassed liquid into a turbine at said opposite end of said rotor; discharging said degassed liquid through said turbine; and discharging said gas through a central gas exhaust.
In the preferred process according to the invention the degassed liquid is discharged peripherically from the turbine at a pumping pressure while the turbine recovers dynamic energy from the degassed liquid.
By using the apparatus and process according to the present invention it is possible to provide an essentially complete separation of the gas which is included in the fluid mixture in gaseous (non-dissolved) form. The gas/liquid separation will be performed with high efficiency and low energy losses.
The present invention also relates to improvements in a process for producing paper or board in a paper machine including the steps of providing a papermaking stock of pulp; diluting said stock in one or more stages with backwater drained through a forming wire of said paper machine; feeding said stock through a head box of said paper machine onto said forming wire; forming a web on said forming wire while allowing water from said stock to drain through said wire; feeding said web through a press section and a drying section of said paper machine to provide paper or board. The improvement comprises pumping at least a portion of said backwater and/or diluted stock with at least one degassing centrifugal pump in accordance with the present invention.
In a preferred embodiment of the invention the backwater is substantially completely degassed in said pump. The pumping of the backwater may be performed with one, or preferably with several degassing centrifugal pumps.
A preferred embodiment of the process is provided by connecting the gas discharge to a vacuum source which may be used either to improve the effect of gas separation in the pump, or as a means of suction in a process upstream of the pump, or as means to provide a pressure drop in the inlet sufficient for distributing the fluid over the periphery of the same.